Cover a flat work area with a large plastic tarp. This makes cleanup easier.
Empty two 10 lb. bags of playground sand onto the center of the tarp. Mound the sand up into a 6-inch tall hill. Shape the sand with your hands to taper the hill to 1/2-inch thick at the edges. If the sand is dry and does not hold its shape, add water. Add 1 cup at a time, stir the sand and water together with your hands. Then, try to mound again. If it is still too dry, add additional water until the right consistency is reached. The sand should be moist and pliable, not soaked.
Cover the mound with a layer of plastic wrap. This keeps the sand off your finished bird bath.
Place a hosta leaf on top of the mold with the top side down. Tear the stem off the leaf. A 13-inch-by-9-inch hosta leaf is a good size, but you can use larger or smaller as desired. You want the veins of the leaf pointing upward so that the bird bath shows the detail. Adjust the mounded sand if necessary. The leaf should rest on top, following the curve of the sand mound. Choose a hosta leaf that is free of holes or tears.
Mix a 10 lb. bag of concrete in a large bucket. Follow the bag's instructions for mixing. You want the cement to be moist and pliable, similar to a pudding.
Transfer the concrete from the bucket onto the veined side of the leaf. Use your hands to place it and then move it around. Cover the surface of the leaf; press the concrete onto it as you go. This helps pick up the vein detail. You want a 3/4-inch thick layer in the center and no thinner than a 1/4-inch layer at the edge.
Leave the concrete to sit and dry, undisturbed, for one week. Remove the set piece from the sand and remove the plastic wrap. Set it back out of the way to dry for another 72 hours before placing it directly on the ground in your garden.